Self-tapping screw and screwed fastening as well as blank for manufacturing the screw

Abstract

A thread-forming screw comprising, at least in part, a shaft (1.1) provided with a screw thread (2) and further comprising a screw tip (1.2) at one end of the shaft (1.1), wherein the shaft (1.1) comprises a thread-forming screw-threaded portion (4) that extends away from the screw tip (1.2) and an adjoining load-bearing screw-threaded portion (3), wherein the load-bearing screw-threaded portion (3) has a round cross-section, characterized in that the load-bearing screw-threaded portion (3) displays, in the direction away from the screw tip (1.2), an increase d in a flank diameter DF and/or an outside diameter DA of the screw thread at a gradient of from 1:50 to 1:500 based on a length LG of the load-bearing screw-threaded portion (3), preferably at a gradient DA or DF to LG of from 1:100 to 1:400, wherein the increase d in the flank diameter DF and/or the outside diameter DA remains constant over the length LG of the load-bearing portion.

Claims

1. A blank for the fabrication of a screw comprising, at least in part, a shaft provided with a screw thread and further comprising a screw tip at one end of the shaft, wherein the shaft comprises a thread-forming screw-threaded portion that extends away from the screw tip and an adjoining load-bearing screw-threaded portion, wherein the load-bearing screw-threaded portion has a round cross-section, wherein the load-bearing screw-threaded portion includes, in a direction away from the screw tip, an increase in at least one of a flank diameter DF or an outside diameter DA of the screw thread at a gradient of approximately 1:50 to 1:500 based on a length LG of the load-bearing screw-threaded portion, wherein the increase in the at least one of the flank diameter DF or the outside diameter DA remains constant over the length LG of the load-bearing portion, the blank comprising: a subsequently to be created thread-forming screw-threaded portion extending away from a subsequently to be created screw tip and an adjoining subsequently to be created load-bearing screw-threaded portion having a load-bearing screw thread to be formed from the blank and other regions to be subsequently formed so as to adjoin the load-bearing portion, wherein the blank has a round cross-section in the subsequently to be created load-bearing screw-threaded portion and a conical shape corresponding to the subsequently to be created load-bearing screw-threaded portion, and wherein the blank includes, for the subsequently to be created load-bearing screw-threaded portion, a cone angle of between 0.1-0.5 that provides a maximum diameter adjacent to a subsequently to be created screw head and a minimum diameter adjacent to the subsequently to be created thread-forming screw-threaded portion.

2. The blank according to claim 1, wherein the blank has a round or trilobular cross-section in the subsequently to be created thread-forming screw-threaded portion.

3. The blank according to claim 1, wherein the screw comprises the thread-forming screw-threaded portion including a trilobular cross-section.

4. The blank according to claim 1, wherein the screw comprises the gradient DA or DF to LG of approximately 1:400.

5. The blank according to claim 1, wherein the screw comprises the thread-forming screw-threaded portion including a cross-section deviating from the cross-section of the load-bearing screw-threaded portion.

6. The blank according to claim 5, wherein the screw comprises the length LG of the load-bearing screw-threaded portion that is equal to at least three times a pitch of the screw thread in the load-bearing screw-threaded portion.

7. The blank according to claim 6, wherein the screw comprises the length LG of the load-bearing screw-threaded portion that is equal to at least approximately 0.8 times a nominal diameter DN of the screw.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The screw of the invention is described below with reference to the drawings, in which:

(2) FIG. 1 shows a screw of the invention comprising a thread-forming screw thread and an accommodating component,

(3) FIG. 2A is a view of a tip of the screw shown in FIG. 1, which tip has a round cross-section,

(4) FIG. 2B is a view of a tip of the screw shown in FIG. 1, which tip has a trilobular cross-section,

(5) FIG. 3 is a diagrammatic illustration of the load-bearing screw-threaded portion,

(6) FIG. 4 is a diagram illustrating the thread flanks;

(7) FIG. 5 shows a first embodiment of the load-bearing screw-threaded portion of the screw shown in FIG. 1;

(8) FIG. 6 shows a second embodiment of the load-bearing screw-threaded portion of the screw shown in FIG. 1;

(9) FIG. 7A shows a blank for the fabrication of the screw of the invention comprising a tip having a round cross-section;

(10) FIG. 7B shows a blank for the fabrication of the screw of the invention comprising a tip having a trilobular cross-section; and

(11) FIG. 8 is a graph comparing the force curves occurring when tightening a screw that is conically shaped and one that is not.

DESCRIPTION OF EMBODIMENT(S) OF THE INVENTION

(12) A screw 1 comprising a thread-forming thread is shown in FIG. 1. The thread 2 worked into a shank 1.1 comprises a load-bearing screw-threaded portion 3 comprising a screw thread having a round cross-section over a length LG and a screw-threaded portion 4 that adjoins the load-bearing screw-threaded portion 3, tapers to the screw tip 1.2 and serves to form a female thread in an accommodating component 10 and has a length Lx as measured from the screw tip.

(13) At the other end of the screw, there is provided a screw head 5 that comprises a contact surface 6 and that is adjoined by a thread runout 7 of the length Ly of the screw thread 2, which thread runout 7 then merges into the load-bearing screw-threaded portion 3. The flank diameter and/or outside diameter GE of the load-bearing screw-threaded portion 3 is largest at this transition point 8.

(14) The flights of the tapered screw-threaded portion 4 are formed with sharp edges and they contribute to the formation of the female thread when they engage the accommodating component 10. The pitch of the thread in the tapered screw-threaded portion 4 is equal to that of the load-bearing screw-threaded portion 3 but the diameter of the tapered screw-threaded portion 4 decreases continuously in relation to that of the load-bearing screw-threaded portion 3 to a final diameter A that is in turn smaller than a diameter C in the accommodating component 10.

(15) The sharp-edged and fully formed thread in the tapered screw-threaded portion 4 makes it easier to start the screw 1 in the accommodating component 10 and commence formation of the engaged flight than when the thread crest is blunted. The length Lx of the tapered screw-threaded portion 4 is from 0.5 to 1 times the nominal thread diameter DN. This results in a gradient of the length Lx to the pitch of from 4 to 5, preferably 4.

(16) The special feature of the thread 2 is the load-bearing screw-threaded portion 3, the flank diameter or outside diameter of which increases continuously from the screw tip to the head in relation to a strictly metric standard unified thread of constant flank diameter or outside diameter, and the flank diameter or outside diameter FE in the transition zone between the load-bearing screw-threaded portion 3 and the tapered thread-forming screw-threaded portion 4 is smaller than the flank diameter or outside diameter GE in the transition region 8 between the load-bearing screw-threaded portion 3 and the thread runout 7. The flank diameter or outside diameter FE that is equal to the nominal diameter thus increases continuously toward the transition region.

(17) The load-bearing portion 3 exhibits an increase d in its flank diameter DF and/or the outside diameter DA over its length LG in the direction extending away from the screw tip 1.2 at a gradient of from 1:50 to 1:500 based on the length LG of the load-bearing portion, preferably at a gradient DA or DF to LG of from 1:100 to 1:400. The increase in the flank diameter DF and/or the outside diameter DA of the load-bearing portion is constant so that the load-bearing screw-threaded portion widens conically as viewed from the screw tip.

(18) The length LG of the load-bearing portion is a multiple of the pitch p of the screw thread in the load-bearing screw-threaded portion 3.

(19) The screwed connection shown in FIG. 1 comprises the thread-forming screw 1 and an accommodating component 10 comprising an opening 11 for reception of the screw. The core hole diameter C of the opening 11 in the accommodating component 10 is larger than the diameter A of the screw tip 1.2, but smaller than the thread diameter FE in the transition zone of a first screw-threaded portion 4 tapering from the metric screw-threaded portion 3.

(20) The screw 1 of the invention makes it possible to effect non-positive engagement of the screw in the accommodating component 10 that furthermore seals the screwed connection even more reliably by means of a increased positive engagement due to the increase in the diameter of the load-bearing screw-threaded portion than in the case of a screwed connection comprising a self-formed cylindrical thread so that the thread is impermeable to gases and liquids even at relatively high pressure differences.

(21) FIG. 2A shows that the thread-forming portion 4-1 can have a round cross-section proceeding from the tip 1.2 and having a radius RA and extending as far as the round cross-section of the load-bearing screw-threaded portion 3 having a radius RFE, or it can have a different cross-section, in this case a trilobular cross-section as shown at 4-2, as shown in FIG. 2B. The load-bearing screw-threaded portion 3 having a round cross-section increases continuously from a radius RFE to a radius RGE independently of the geometry of the tip 1.2 and that of the thread-forming portion 4.

(22) FIG. 3 is a diagrammatic illustration of the conical shape of the load-bearing screw-threaded portion 3. Starting from the smaller diameter FE, the diameter increases at a cone angle alpha over the length LG to the diameter GE.

(23) Various embodiments of the screw thread are shown in FIGS. 4 to 6. FIG. 4 shows the basic construction of a single flight comprising a thread crest 21 and a thread root 22, the flank diameter 23 being located precisely therebetween, as shown by the dashed line.

(24) In FIG. 5, the flank diameter 23-1 increases continuously with the diameter of the thread root 22-1 remaining constant, since the diameter of the thread crests 21-1 increases, and thus the thread height, that is, the difference between the thread crest 21-1 and the thread root 22-1, increases.

(25) In FIG. 6, the flank diameter is denoted 23-2, the thread height remains constant, and the diameter of the thread root 22-2 increases at the same cone angle as that of the thread crests 21-2. Since the cone angle is very small, that is, in the range of 0.1-1.5, it is possible to produce both types of threads without any noticeable alteration in the appearance of the metric screw.

(26) FIG. 7A shows a blank 31 for the fabrication of a screw, which blank comprises a shank 32-1 comprising a subsequently to be created thread-forming portion 34 extending away from a subsequently to be created screw tip 31.2-1, a subsequently to be created load-bearing screw-threaded portion 33 adjoining the thread-forming portion 34 and comprising a load-bearing screw thread, and optionally additional portions adjoining the subsequently to be created load-bearing screw-threaded portion. The blank 31 has, at least in the subsequently to be created load-bearing screw-threaded portion 33, a round cross-section and a conical shape corresponding to the screw to be produced and having the angle alpha so that the cross-section of the subsequently to be created load-bearing screw-threaded portion 33 increases continuously from the end adjoining the thread-forming portion 34 to the end adjacent to the head 35.

(27) Furthermore, the cross-section of the subsequently to be created thread-forming screw-threaded portion 34 is likewise round in shape and tapers to the tip 31.2-1.

(28) FIG. 7B shows another embodiment of the thread-forming portion 34 of the blank 31, namely, one having a trilobular cross-section likewise tapering towards the screw tip 31.2-2, and a shank 32-2, while the load-bearing screw-threaded portion 33 is designed as shown in FIG. 7A.

(29) The diagram shown in FIG. 8 illustrates the qualitative relationship between the torque and the turning angle of the screw when tightening the screw in the form of a directly screwed connection in a through hole of an accommodating component without intrinsic flights. The torque is given in Nm, the turning angle is given in degrees and is also equal to one tap depth in the case of a directly screwed connection.

(30) The figure shows three curves K1, K2, K3, of which the lowermost curve K1 corresponds to a commercially available thread-forming screw and the curves K2 and K3 located above the lowermost curve K1 correspond to a thread-forming screw of the invention comprising a conically formed load-bearing screw-threaded portion. The curve K2 is distinguished from the curve K3 by the cone angle of the conically formed load-bearing screw-threaded portion, the cone angle of the curve K3 being larger than that of the curve K2. The required self-retention of the screwed connection depending on its application can be effected by the selection of a suitable cone angle for the screw, which is illustrated by the hatched region B, which shows a field of various cone angles and the associated curve, the region B being delimited by the curves K2, K3.

(31) The three curves K1, K2 and K3 are each divided into three sections I, II, and III, as plotted against the turning angle of the screw. The characteristic of the forming torque is shown in the first portion I and all three curves K1, K2, and K3 are disposed one above the other and exhibit, in this case, a substantially linear increase in torque from the initial engagement of the thread-forming portion to the point at which the maximum forming torque F is attained on reaching the largest thread diameter of the thread-forming portion. The maximum forming torque F is reached at a turning angle D1. When the screw is tightened, there takes place a plastic and elastic deformation of the accommodating component with flights being formed in the accommodating component. The torque increase itself depends on the geometry of the thread-forming portion and can deviate from the linear course. The forming torques of a round thread-forming portion and a trilobular thread-forming portion also differ from each other with the latter usually being lower. For reasons of comparability, the thread-forming portions of the screws for which the curves K1-K3 are relevant are of identical shape.

(32) In the adjoining section II, the thread-forming portion has passed through the through hole in the accommodating component and, as the turning angle increases further, there takes place a plastic deformation of the thread formed in the accommodating component which may be regarded as a smoothing effect. As soon as the smoothing effect is accomplished, which is the case before a turning angle D2 is reached, there remains only the clamping torque MK generated by the elastic recovery reaction of the deformed female thread, and this clamping torque MK results in a clamping effect or self-retention of the screw. It is clear that the clamping torque of the curve K1 corresponding to a commercially available thread-forming screw is lower than the clamping torque of the thread-forming screws of the invention comprising a conically formed load-bearing screw-threaded portion, while the clamping torque of the curve K2 is higher than that of the curve K1 but lower than the clamping torque of the curve K3 and still lower than the maximum thread-forming torque F. However, the clamping torque of the curve K3 is higher than the maximum thread-forming torque F.

(33) In the case of a small cone angle, the smoothing effect of the flights formed in the accommodating component as the turning angle increases, that is, with further tightening of the screw, is compensated, at least in part, by the increase in cross-section. In the case of a larger cone angle, there will additionally take place displacement of material and thus a further increase in the clamping effect and the clamping torque.

(34) The flights of the female thread in the accommodating component bear positively against the flights of the thread-forming screw to provide a sealing effect that is a distinct clearly improvement on the commercially available thread-forming screws.

(35) In section III, further tightening of the screw after a turning angle D2 has been reached, results in the screw head coming to bear against the accommodating component or an accommodated component located therebetween, which results in locking of the screw. When the turning angle increases beyond this point, the accommodating component usually breaks down and the screw damages the female thread. The curves K1, K2, and K3 thus end in section III when the screwed connection fails on reaching over-torque, also referred to as torque at failure. The ends of the curves K1, K2, and K3 are located one above the other, that is to say, the over-torque of the screwed connection also increases as the cone angle increases.